Implement Clone and DeepClone for extra::time::{Tm, Timespec}

I need `Clone` for `Tm` for my latest work on [rust-http](https://github.com/chris-morgan/rust-http) (static typing for headers, and headers like `Date` are a time), so here it is.

@huonw recommended deriving DeepClone while I was at it.

I also had to implement `DeepClone` for `~str` to get a derived implementation of `DeepClone` for `Tm`; I did `@str` while I was at it, for consistency.